1. Field of the Invention
The present invention relates generally to mass airflow sensors and, more particularly, to a mass airflow sensing system arranged in association with a conduit in which a plurality of ports are disposed around the inner circumference of the conduit and connected in fluid communication with the mass airflow sensor.
2. Description of the Prior Art
Airflow sensors are used in many different types of applications, such as heating and ventilating systems, medical applications and automotive systems. For example, U.S. Pat. No. 4,885,938, which issued to Higashi on Dec. 12, 1989, discloses a flow meter fluid composition correction system. It utilizes certain relationships among the mass flow signal, the thermal conductivity, the specific heat and the density of a fluid being measured to correct a mass flow meter measurement with respect to changes in composition of the fluid that is being measured.
U.S. Pat. No. 4,864,724, which issued to Bergstrom on Sep. 12, 1989, describes a planar mounting of silicon micromachined sensors for the purpose of measuring pressure and fluid flow. This patent describes a process for flush mounting a silicon micromachined sensor for fluid flow measurements with an adjacent face of a substrate. The faces of the substrate and the sensor, over which the fluid flow passes, are disposed on a conformal surface. An adhesive is applied between the sensor and the substrate to secure the sensor and substrate to each other. Upon removal from the conformal surface, the faces are cleaned of adhesive and the electrical signal generating means of the sensor are connected with signal processing means, such as by wire bonding. The sensor lies flush with the face of the substrate, avoiding interference with the fluid flow over the sensor face and, therefore, maintaining sensitivity throughout the life of the sensor.
U.S. Pat. No. 4,856,328, which issued to Johnson on Aug. 15, 1989, describes a sampling probe flow sensor which comprises a tubular probe body that is located within and aligned with a venturi passageway. The probe body is provided with a channel therethrough connecting openings located at high and low pressure regions in the venturi passageway. The openings are configured to cause a least a 90 degree change in direction of air flowing therethrough from the venturi passageway to protect a fragile flow sensor in the channel from damage by suspended contaminants.
U.S. Pat. No. 4,776,214 which issued to Moran et al on Oct. 12, 1988, describes a mass airflow sensor which comprises a sensing element for the mass airflow sensor disposed on a first portion of a thin flexible film substrate while an extension of the substrate provides integral electrical conductive metallizations that connect the sensing element to associated sensor electronic components mounted on a second portion of the film substrate. This device eliminates the need for individual soldered wire connections to connect the sensing element to its associated electronics.
U.S. Pat. No. 4,749,122, which issued to Shriver on Jun. 7, 1988, discloses a combustion control system in which the economic optimum efficiency is achieved by periodically computing a relative index of combustion efficiency using the combustion chamber as a real-time, on line calorimeter. This is accomplished by first determining the combustion chamber inputs and outputs required to calculate a relative index of performance, sequentially comparing the latest relative index value with the previous value and then adjusting the air-to-fuel ratio to achieve an improved index value.
U.S. Pat. No. 4,431,132, which issued to Edmundson on Feb. 14, 1984, describes a control valve system for gas water heaters in which a valve is provided with a snap action characteristic and is constructed to inhibit the supply to the burner of insufficient amounts of gas to ensure reliable ignition. For example, a flexible diaphragm can be provided with opposite sides that are connected to respective passages to receive pressures upstream and downstream of a restriction in a water flow path. The diaphragm can open a gas valve in response to a predetermined flow of fluid along the path. In order to provide the snap action characteristic, a further passage is provided that links the two passages coupled to the diaphragm and contains a ball that can block the further passage only when the fluid flow is sufficient to open the gas valves sufficiently for an ignitable quantity of gas to flow to the burner.
A paper by P. W. Foss, entitled "A Novel Design of Viscous Flowmeter", was delivered at the International Conference on Advances in Flow Measurement Techniques in September, 1981. This paper describes the development of a viscous flow meter which uses two commercially available flame traps connected in series and separated by a small airspace. The flow meter is inserted in a standard pipe. Experimental results are presented in this paper which show that the flow meter behaves as a conventional laminar flow meter. The flow meter was designed for use with a variable compression engine and was evaluated during the production testing of several engines.
A paper, by K. A. Blake, entitled "The design of piezometer rings", was printed in the Journal of Fluid Mechanics, Volumn 78, part 2, pages 415-428 which Was printed in Great Britian in 1976. This article describes the theoretical work performed at the National Engineering Laboratory in Glasgow, Scotland. A mathematical model is discussed in which a reading from a piezometer ring is predicted from the individual wall static measurements and, from the mathematical model, the conventional form of a piezometer ring is shown as being unreliable in asymmetrical flow. The paper also describes an alternative form of ring known as the Triple-T which is analyzed and shown to provide a true average pressure in all cases. The paper recommends that the Triple-T form be used in future tests. Throughout the paper, a description of piezometric devices is provided in terms of its use to measure pressure at various places along a flow channel.